1. Field of the Invention
The present invention relates generally to electron emission materials and articles, and their methods of manufacture; and more particularly to materials and articles with high melting points, low vapor-pressures, high thermal and electrical conductivity, low work functions, and high photon and electron emission properties for use as electrodes in high intensity light sources, gas lasers, electron-beam lithography equipment, welding and plasma cutting torch applications, x-ray and microwave generators, and other electromagnetic apparatus.
2. Description of Related Art
In the field of high-power electronics, including lighting (arc-lamps, flash-lamps, and gas lasers), arc-welding and plasma-cutting, electron-beam lithography, and other instrumentation (e.g., electron microscopy, spectroscopy, x-ray generators), electrodes are used to provide high energy density streams of electrons. In the case of direct-current (DC) applications, the cathode is the negatively charged electrode and dispenses electrons to the positively charged anode that receives the electrons and conducts them away. In alternating-current (AC) applications, the electrodes alternate from being cathodes to anodes with the alternating current at different intervals, or frequencies. The stream of electrons produced from the cathode provides the energy necessary to perform work in the form of heating, electron emission, elemental or chemical ionization, or collisions to produce other forms of electromagnetic energy like x-rays, ultraviolet radiation, infrared radiation, and microwave radiation.
Electrode materials require good chemical, mechanical, and electrical properties to perform well for extended periods of time. Typically, DC arc-lamps and DC arc-welding electrodes are made of tungsten (W) and thoriated tungsten (e.g., W+2% ThO.sub.2); DC flash-lamp electrodes are made of tungsten and porous tungsten that has been infiltrated with an emitter material like strontium barium calcium aluminate; for instruments like electron microscopes, the cathode is made of single crystal lanthanum hexaboride (LaB.sub.6) or cerium hexaboride (CeB.sub.6); and for other types of analytical instruments that use electron-beams, the cathodes, or field emission devices can be made of zirconiated tungsten (W+ZrO.sub.2-x).
In the case of DC cathode applications (e.g., arc-lamps, arc-welding), thoriated tungsten is used almost exclusively. The cathodes are made of tungsten doped with approximately 2 percent thorium dioxide (W:2% ThO.sub.2). Tungsten serves as the refractory metal-matrix which has a very high melting point, it is very electrically and thermally conductive, has reasonably good thermionic emission properties, yet has a work function of approximately 4.5 eV when pure. Thorium dioxide (thoria) is the most refractory oxide ceramic material known (highest melting point and lowest vapor-pressure), and when properly added in small amounts (typically 1 to 3%) to tungsten, thoria aids in controlling the tungsten microstructural characteristics by "pinning" grain boundaries, thereby inhibiting exaggerated or non-uniform grain growth. Further, these characteristics, along with other properties provided by the thoria, lower the work function of the metal-ceramic system to approximately 2.7-3.0 eV. The lower work function enables the W:2% ThO.sub.2 cathode to emit thermionic electrons at lower temperatures and with less localized heating at the tip; thus, the thoriated tungsten electrode maintains its integrity longer than pure tungsten would without the thoria additive.
Recently, there has been much effort expended investigating alternative materials for replacing thoria in tungsten. This is due to the fact that thoria is radioactive and is considered a carcinogen (a hazardous material), its decay products are toxic, and it is likely to become a strictly regulated and controlled substance. Some headway is being made by doping tungsten with other oxides like lanthana (La.sub.2 O.sub.3), yttria (Y.sub.2 O.sub.3), ceria (CeO.sub.2), and mixtures of these.
In the case of DC flash-lamp cathodes, these are typically called "dispenser" cathodes, and are made by infiltrating porous tungsten (approximately 80% dense, or 20% porosity) with mixed oxides in their molten state. Once cooled (frozen), the materials are machined to form the finished component. The oxides used generally consist of aluminates, and are engineered to not only have very low work functions with good electron emission, but reasonably low melting points for convenience in manufacturing. The term "dispenser cathode" is literally accurate in that they "boil-off" not only electrons, but the chemicals that make them-up. Some of the better-known aluminates are: barium calcium aluminate (in proportions: 3BaO:1CaO:1Al.sub.2 O.sub.3); barium strontium aluminate (in proportions 3BaO:0.25SrO:1Al.sub.2 O.sub.3); or other combinations of strontium oxide, barium oxide, calcium oxide, and aluminum oxide. These types of dispenser cathodes work well for the present-day applications, for the most part, yet in higher-power loadings, they sputter-off (caused from melting or boiling of the oxides) some of their constituents. These sputtered materials cause the inside of a flash-lamp envelope to become clouded (or dirty) thus lowering the output of light from the lamp. Also, "sputtering" is a form of erosion and is the primary cause of cathode and lamp (or instrument) failure.
In the case of analytical instruments like electron microscopes, these employ cathodes made of single crystal lanthanide hexaboride (e.g., LaB.sub.6, CeB.sub.6). Instruments such as these utilize thermionic electron-beams, generated at .apprxeq.1600.degree. C., at very high voltages (10-20 kV) and very low current (10.sup.-6 amp to 10.sup.-10 amp). These single crystal materials work well for certain applications, but are very expensive, and only one or two suppliers for these cathodes exist. Also, these single crystal cathodes are susceptible to thermal shock during cyclic, high power loadings. When tested in multi-kilowatt arc-lamps that run at low voltage (.apprxeq.20V to 50V) and high current (50 A to 100 A), the LaB.sub.6 materials cracked (shattered) due to thermal shock caused by very-rapid resistance heating. In addition to this example, single crystals and polycrystalline ceramics of binary carbides like zirconium carbide (ZrC), hafnium carbide (HfC), tantalum carbide (TaC), and others, have been experimented with for use as thermionic cathodes in analytical instruments. Although some of these have displayed promise for high voltage--low amperage applications, when these binary carbide materials were tested under cyclic, high-power loadings (multi-kilowatt, low voltage x high amperage), they consistently exhibited cracking and failure due to thermal shock; in addition, they are difficult to produce and expensive.
In the case of field emission devices used for electron-beam lithography and analytical instruments, the cathodes are sometimes made of tungsten that has been doped or coated with an emitter material like zirconium oxide, thus the term "zirconiated tungsten". These cathodes can be operated as "cold-field" emitters, or "hot-field" emitters; as the terms imply, one is operated at ambient temperatures where the other is heated (resistively) to an emission temperature of approximately 1800.degree. C.
The different applications (short-arc lamps, long-arc flash-lamps, welding and cutting, and electron-beam analytical instruments, etc.) utilize different power supplies that produce different power loadings in the electrodes. The examples given above describe a few of low voltage--high amperage applications, and high voltage--low amperage applications. Each of these examples utilize different cathode materials and geometries.
There is a need for better materials and electrodes that offer higher performance and longer-life, with properties allowing for flexibility to be used in different types of electron emission applications.